Syrian opposition leaders have reacted sceptically to a joint call by the US and Russia for an international conference to discuss the creation of a transitional government in Damascus to end the country's escalating 25-month crisis.

Moaz al-Khatib, who resigned last month as head of the National Opposition Coalition (NOC), the main western- and Arab-backed grouping, warned: "Syrians: be careful of squandering your revolution in international conference halls."

Walid Saffour, the NOC's London representative, said he was sceptical, though a formal decision had yet to be taken. Previous calls for negotiations with President Bashar al-Assad have caused bitter divisions in opposition ranks and been flatly rejected by armed groups.

John Kerry, the US secretary of state, and Sergei Lavrov, his Russian counterpart, issued the call in Moscow on Tuesday after months of deadlock over Syria's bloody crisis. Officials from both sides hope representatives from the regime and opposition will attend. Russia remains Assad's staunchest ally and has opposed foreign involvement in the conflict, which has killed more than 70,000 people since it erupted in March 2011.

The vaguely worded proposal for new talks raises questions over calls in the US to arm the Syrian rebels – a position that is opposed vigorously by the Syrian government and by Russia. Kerry said that if negotiations were held, "then hopefully that would not be necessary".

It also creates a dilemma for Britain and France, which have proposed lifting or amending the EU arms embargo on Syria to help the anti-Assad rebels but have met opposition from other member states.

The US-Russia agreement was warmly welcomed on Wednesday by the joint UN-Arab League envoy to Syria, Lakhdar Brahimi, whose diplomacy has been stalled for months by divisions in the UN security council. "This is the first hopeful news concerning that unhappy country in a very long time," said his office. "The statements made in Moscow constitute a very significant first step forward. It is nevertheless only a first step."

The idea is based on the UN-convened conference held in Geneva in June last year, which left open the key question of what, if any role, would be played by Assad in a transitional government. The Syrian president has said repeatedly that he has no intention of standing down. The Geneva communique also called for an immediate cessation of violence.

"Before making any decisions we need to know what Assad's role would be," warned the NOC's Samir Nashar. "That point has been left vague, we believe intentionally so, in order to try to drag the opposition into talks before a decision on that is made. No official position has been decided but I believe the opposition would find it impossible to hold talks over a government that still had Assad at its head."

Suspicious responses by other observers and opposition supporters appeared to herald controversy over who would take part in talks and under what conditions. "What 'opposition' has the credibility and will to negotiate with the Assad regime?" asked Salman Shaikh of the Brookings Institution in Doha. "Geneva II is dead on arrival."

Colonel Qassim Saadeddine, a spokesman for the rebel Supreme Military Council, said the armed opposition would not get involved. "Unfortunately I don't think there is a political solution left for Syria," he told Reuters. "I think that is clear by now. We will not sit with the regime for dialogue. And frankly, I don't think Assad's decisions are really in Russia's hands. Right now he is only looking toward Iran."

In other developments, rebel sources reported the fall of Khirbet Ghazaleh, a key town in southern Syria, to government forces, because weapons the rebels had hoped would be delivered from Jordan had not arrived.